1. Field of the Invention
This invention relates to substituted lithium phosphates and more particularly to lithium salts of zirconium and hafnium phosphates and to their use as solid electrolytes in electrochemical cells.
2. Description of the Prior Art
Solid ionic conductors are desirable for use as solid electrolytes.
Li has long been regarded as a desirable component of galvanic cells. It is inexpensive and its high reduction potential (E.degree..sub.red =- 3.024v) and light weight have often suggested its use as the anodic component in high energy-density storage batteries. (See, for example, S. Pizzini, J. Appl. Electrochem 1, 153 (1974)). The reactivity of Li with water has required the use of non-aqueous electrolytes such as organic soluble lithium perchlorates, or fused lithium halides. Li.sup.+ conductors which are solid and are more highly conductive at lower temperatures than previously used electrolytes would be quite useful in such utilities; these conductors also serve to separate the anodic and cathodic components.
Solid electrolytes and in particular completely solid state galvanic cells offer special advantages as low-energy density, low-power density primary batteries. These advantages include the possibility for long shelf life, broad temperature limits of operability and miniaturization. A Li.sup.+ conducting solid electrolyte would provide the basis for a lighter and lower cost alternative to the solid state batteries which rely on the transport of Ag.sup.+ in various silver halides.
The known Li.sup.+ conductors with the highest conductivities are LiI mixed with CaI.sub.2 (C. R. Schlaiker and C. C. Liang, Proc. NATO Adv. Study Inst. of Fast Ion Transport, Sept. 5, 1972); single crystals of lithium .beta.-alumina LiAl.sub.11 O.sub.17 (M. S. Whittingham and R. A. Huggins, NBS spec. Pub. 364, Solid State Chemistry, 139 (1972) and single crystals of certain of the Cl-Br lithium haloboracites (Bither and Jeitschko, U.S. Pat. No. 3,911,085). All have conductivities at 200.degree. C of the order of 10.sup.-.sup.3 (.OMEGA.- cm).sup.-.sup.1.
Two recently issued Japenese patents discuss the phases Li.sub.2 SO.sub.4.sup.. LiOH.sub.. LiI and Li.sub.2 SO.sub.4.sup.. LiOH with conductivities of 5 .times. 10.sup.-.sup.2 (.OMEGA. cm).sup.-.sup.1 and 1.5 .times. 10.sup.-.sup.2 (.OMEGA. cm).sup.-.sup.1, respectively at 200.degree. C (Japanese Kokai: 49-81899 and 49-81898, issued Aug. 7, 1974). However, these phases appear to melt below 200.degree. C and therefore are not useful as solid electrolytes at a temperature of 200.degree. C or above.
The preparation of the phosphates M.sup.I M.sub.2.sup.IV (PO.sub.4).sub.3 where M.sup.I = Li,Na,K,Rb,Cs and M.sup.IV = Zr, Hf is described by M. Sljukic, B. Matkovic, B. Prodic and S. Scavnicar, Croat. Chim. Acta. Zagreb, 39, 145-8, (1967). Lattice constants were obtained from oscillation and Weissenberg X-ray diffraction photographs. The X-ray data suggest that all these compounds are isostructural with space group R3C or R3C. These compounds were prepared by heating the reactants at a maximum temperature of 1200.degree. C and then cooling at a slow rate. L. Hagman and P. Kierkegaard, Acta. Chem. Scand. 22, 1822 (1968), report the results of a detailed crystal structure determination of NaZr.sub.2 (PO.sub.4).sub.3. It was found to have the space group R3C. The structure consists of a three-dimensional framework of PO.sub.4 tetrahedra and ZrO.sub.6 octahedra which are linked by corners. The sodium ions have six fold coordination, although these octahedra are severely distorted.
No conductivity measurements have been reported in the above references for LiHf.sub.2 (PO.sub.4).sub.3, LiZr.sub.2 (PO.sub.4).sub.3, NaHf.sub.2 (PO.sub.4).sub.3, NaZr.sub.2 (PO.sub.4).sub.3 or any of the other known end member compositions M.sup.I M.sub.2.sup.IV (PO.sub.4).sub.3, where M.sup.I = Li, Na; M.sup.IV = Ti, Ge, Sn, Th, U.